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Physical Model and Design Rules for the Optimization of Solar Chimney Systems

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Physical Model and Design Rules for the Optimization of Solar Chimney Systems U. Dietrich, Int. J. of Energy Prod. & Mgmt., Vol. 3, No. 4 (2018) 307–324 PHYSICAL MODEL AND DESIGN RULES FOR THE OPTIMIZATION OF SOLAR CHIMNEY SYSTEMS UDO DIETRICH REAP research group (Resource Efficiency in Architecture and Planning), HafenCity University Hamburg, ­Germany. ABSTRACT Natural ventilation is a basic quantity to reach comfort in passively acting buildings. It delivers not only fresh air to breath but can also be used to temper the room if the indoor temperature is above the outdoor one. Driving forces are temperature differences (buoyancy) and wind. However, both may be weak in hot and especially humid locations. A solar chimney uses solar radiation to heat up the exhausted air and to increase buoyancy, thus could help to improve that situation at least during the daytime. Nevertheless, the implementation of solar chimneys is quite rare. It may be that the idea to use heat to cool and ventilate a building seems strange. The literature reports about the potential of solar chimneys, characteristics like vol- ume flow and temperatures are measured or simulated. Though, the findings of these publications are based on a special geometry and provide not enough detailed information about the optimized shape (width, height, length, etc.) of solar chimneys. To overcome that situation, this paper presents general design rules for the geometry of solar chimney systems that could be adapted to existing or newly erected buildings. A solar chimney system is assumed as composed by three components: An absorber as the main part to reach a higher temperature, a stack extension on top for further accel- eration of the exhausted air, and a stack at the bottom connected to the (lower) storeys that should be ventilated. A full set of equations of fluid mechanics and thermodynamics is presented and describes the physi- cal behaviour of the air in the system. These equations are coupled with each other and can be solved in iterations, also with a simple spreadsheet. As a compromise between accuracy in results and complexity of the simulation method, the physical model is based on some simplifying assumptions like turbulent flow, friction on walls, air as an incompressible medium, and immediate heat transfer from the absorber to the air. Nevertheless, the outcomes are in accordance with the findings from the literature, as the model seems to reflect the physical behaviour adequately. The main results are volume flow, velocity and temperature, allowing the optimization of the geometry of the chimney system. An applicable list of design rules for solar chimneys is finally presented as well as proposals for their integration in typical apartment buildings in hot and humid locations. Keywords: application in apartment buildings, buoyancy, design rules for geometry of solar chimneys, friction, hot and humid Climates, natural ventilation, physical model, solar chimney. 1 NATURAL VENTILATION IN HOT AND HUMID CLIMATES 1.1 Functions of natural ventilation and specialities in hot and humid climates Natural ventilation has the main purpose to remove CO2 (and replace it by O2), humidity and heat (and thus cooling with outdoor air). Driving forces for natural ventilation are the differ- ences between indoor and outdoor in pressure (wind) and temperature (buoyancy). In hot and humid climates, sufficient ventilation is especially difficult to reach. Because of the small changes in temperature between day and night, there is nearly no temperature dif- ference between in- and outdoor. For many of the locations, wind velocity is quite low supplementary thus both principles do not work strongly. © 2018 WIT Press, www.witpress.com ISSN: 2056-3272 (paper format), ISSN: 2056-3280 (online), http://www.witpress.com/journals DOI: 10.2495/EQ-V3-N4-307-324 308 U. Dietrich, Int. J. of Energy Prod. & Mgmt., Vol. 3, No. 4 (2018) 1.2 Vernacular architecture The first priority was a comfortable temperature indoor during the night for sleeping. During the day, people stayed outside. That could be realized best with a light construction that does not store the heat of the day and allows that indoor temperature drops quickly with outdoor temperature (see also chapter 5 for thermal behaviour). It is known from practical experience that even if there was a vertical opening (like the eye in a staircase and an opening in the roof) offering a ventilation stack for buoyancy, the air rested on the bottom of the building and does not move upwards. As a consequence, vernacular architecture concentrated on the wind as driving force. Walls (and even floors!) were open for a maximum of air movement, using woven structures as the principal ones. In cold climates, each oven produced hot air in a chimney. As a result, strong buoyancy was created, and the smoke was removed out of the building. This idea to use a heat source (like the sun) to produce hot air in a ventilation stack was not developed in hot climates, may be because of the lack of material (glass) and experience, or maybe because of the odd concept of heating something to cool a building. Supplementary, the sun as heat source works only during the day and not during the night, when the need was the strongest. 1.3 Today’s conditions and needs Architecture today is different and widely not following the principles of vernacular architec- ture. Walls become tighter and not permeable for air movements, windows have a glazing, also for (sound) protection. Especially in towns, buildings are placed near together and mul- ti-storey, creating obstacles for wind. Technical HVAC equipment undertakes increasing tasks of ventilation and tempering of the rooms. On the other hand, there are a lot of countries where inhabitants have not enough financial means to pay for the supplementary energy demand and power cuts may happen. As a result, inhabitants live often under worse condi- tions than their ancestors in vernacular architecture. But there are also chances: the existence of a wider range of building materials like glazing and thermal insulation allows the construction of solar chimneys. Especially for residential buildings in locations with a rural character, such equipment can help a lot, especially since it does not cause permanent energy costs. People stay more and more in their apartments also during the day, when the system works best. This paper is thought as a contribution for passive optimization of buildings. Solar chim- neys could be a reasonable supplement for today’s buildings in hot and humid climates. The given rules of thumb for design shall help to introduce them to contemporary architecture. 2 PRINCIPLES OF A SOLAR CHIMNEY 2.1 Absorber To facilitate the model, the absorber is assumed with a rectangular cross-section (assume other shapes as a rectangle with an identical area!) with a width W and a depth D (Fig. 1). Three sides of the absorber are assumed as adiabatic (no heat flow). They could be integrated into the building or not and are assumed as with thermal insulation and coloured black to absorb incoming solar radiation. The fourth side is oriented to the sun and has a glazing that separates the air volumes inside and outside of the absorber. U. Dietrich, Int. J. of Energy Prod. & Mgmt., Vol. 3, No. 4 (2018) 309 Figure 1: Horizontal cross-section of the absorber of a solar chimney. The sun is shining through the glazing onto the absorbing walls. They absorb this radiation and heat up finally the air in the absorber. Because of buoyancy, the air in the absorber moves up and is accelerated according to the passed height difference. 2.2 Stack extension It may be that the air at the end of the absorber has still a temperature difference with the outdoor temperature, which means that the acceleration potential is still not completely used. A simple vertical stack extension without glazing can serve with its height difference to accelerate the air further (Fig. 2). It is assumed to be rectangular with a depth d, a width w and vertical with a height h. All four walls are opaque and oriented to outdoor conditions and thus outdoor temperature. To avoid quick thermal losses and thus temperature reduction as well as an early end of the acceleration, the walls of the stack extension have a thermal insulation. 2.3 Stack In a multi-storey building, there are also floors below the absorber. A supplementary stack can be added on the bottom of the absorber connecting downwards to the floor to ventilate (Fig. 2). It is assumed as rectangular with width w’, depth d’ and height h’. The stack is Figure 2: Complete solar chimney system with stack, (sloped) absorber and stack extension, and the corresponding volume flow (V), temperatures (T) of the air, velocities (v) and times (t). After the time t3, the air is exhausted on top of the system. The lower end of the absorber is connected to the stack and drawing air out of the building, the upper one transfers the air to the stack extension and finally to the outside. 310 U. Dietrich, Int. J. of Energy Prod. & Mgmt., Vol. 3, No. 4 (2018) integrated completely into the building. There is no temperature difference between the stack and the surrounding air, thus there are neither heat losses nor heat gains. The air must be drawn upwards by the absorber causing a supplementary load for it. 2.4 The complete solar chimney system: Construction and operating principle The absorber is often integrated into sloped roofs: that’s why it may have a slope. Length L and slope result in a height difference H (Fig. 2). The accelerating forces act in the absorber and in the stack extension. But there are also forces hindering the air movement: • The friction of moving air on the walls (growing with the square of the velocity!).
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